Ultrasonic surgical instruments are known which include ultrasonic surgical blades. A handpiece of a known ultrasonic surgical instrument includes an ultrasonic transducer which is powered by an ultrasonic generator through a cable. An ultrasonic transmission rod of the instrument has a proximal end and a distal end, wherein the proximal end is operatively connected to the ultrasonic transducer. An ultrasonic surgical blade is activated by the distal end of the ultrasonic transmission rod. Known blade shapes include straight blades and curved blades and include blades that are symmetric and blades that are asymmetric about a longitudinal axis or about a curved centerline of the blade.
A known ultrasonic surgical blade is a cylindrical blade which has a distal tip, a most-distal vibration node (a vibration node being a point of substantially zero displacement), and a second most-distal vibration antinode (a vibration antinode being a point of maximum displacement relative to all other points in a half wave), wherein the most-distal vibration antinode is the distal tip. Longitudinal ultrasonic vibration of the blade generates motion and heat in the contacted tissue, wherein the heat primarily provides the means for the blade to cut and/or coagulate patient tissue. The blade has a gain step located a distance from the most-distal vibration node which is less than 5% of the distance between the distal tip and the second-most-distal vibration antinode because locating the gain step close to the most-distal vibration node maximizes the vibration amplitude gain. The known blade consists of a larger-diameter right-circular geometrically-solid cylinder from the second most-distal vibration antinode to the most-distal vibration node. The known blade consists of a smaller-diameter right-circular geometrically-solid cylinder from the most-distal vibration node to the distal tip. The change in diameter provides a gain in vibration amplitude for the smaller-diameter section of the blade equal to the ratio of the transverse cross-sectional areas of the larger diameter blade section to the smaller diameter blade section when the gain step is located at the node.
The active length of an ultrasonic surgical blade is defined by applicants as the distance from the distal tip to where the vibration amplitude (i.e., the longitudinal vibration amplitude) has fallen to 50% of the tip amplitude. The blade is not considered useful beyond its active length. The active length is about 15 mm for a straight cylindrical titanium rod at a resonant frequency of about 55.5 kHz.
It is known in ultrasonic welding of plastics to provide an ultrasonic welding rod having a gain step, such as a discontinuity between a larger and a smaller rod diameter, which is located between the most-distal vibration node and the distal end of the welding horn and which is spaced apart from the most-distal vibration node of the welding rod by a distance less than 5% of the distance between the second-most-distal vibration antinode and the distal end of the welding rod. It is also known in ultrasonic welding of plastics to provide an ultrasonic welding rod with a hole or a slot to provide a gain in longitudinal vibration amplitude.
What is needed is an improved ultrasonic surgical blade, and an improved ultrasonic surgical instrument which includes an ultrasonic surgical blade, having a longer or shorter active length.